1. Field of the Invention
The present invention relates to an electromagnetic-induction heater which is adapted to heat a continuous thin sheet due to electromagnetic induction.
2. Description of the Prior Art
This type of electromagnetic induction heater is generally arranged to heat a thin sheet having a thickness smaller than its breadth such as a strip (hereinafter referred to simply as "strip") by the use of electromagnetic induction. This known type of electromagnetic induction heater is equipped with electromagnets which are disposed in a face-to-face relationship with and transversely to the direction of travel of a strip, and the electromagnets are energized by an alternating current. It is desirable that the heater is capable of uniformly heating the strip transversely relative to the length thereof, and in addition, it is suitable that, even if the strip is varied in breadth, uniform heating can be effected on the strip.
To meet the above described demands, proposals have heretofore been made of a heater in which, if the breadth of a strip to be heated is varied, electromagnets can be replaced in correspondence with the breadth of the strip, or alternatively, complicated coils are wound around the poles of electromagnets for the purpose of finely adjusting currents flowing in the coil windings so as to cause uniform temperature distribution all over the breadth.
However, these prior-art heaters have a disadvantage in that they need a complicated structure, and cannot rapidly cope with modification of or variations in the breadth of the strip. Particularly, in cases where electromagnets are to be replaced or temperature distribution becomes nonuniform due to changes in external conditions occurring during operation, the prior-art heaters also involve a disadvantage in that the readjustment of electromagnets cannot be easily and quickly performed.
To solve such disadvantages, a heater is known in which each electromagnet is constituted by a plurality of magnetic-pole segments which are disposed in a series which is transverse relative to the length of the strip and can be individually moved toward the strip and a common coil surrounding the segments. This type of the electromagnetic induction heater is disclosed in U.S. Patent Specification No. 3,444,346. The heater is capable of rapidly coping with the occurrence of nonuniformity in temperature distribution by individually adjusting the gap between the plurality of magnetic-pole segments and the strip. However, even this type of heater cannot reduce the fluctuation in temperature distribution to less than 10 percent near the edges of the strip. To improve this situation, the specification of Japanes Patent Unexamined Publication No. 205183/1984 proposes another heater which is arranged to accurately obtain the distribution of a magnetic field, which is required in theoretical terms, by providing plate-shaped side magnetic-pole elements made of a magnetic material in contact with a yoke of the above constructed electromagnets which are located near the breadthwise ends of the strip. The side magnetic-pole elements are provided to guide excessive flux extending out of the breadthwise ends of the strip and prevent it from affecting the strip. As the result, deterioration of power efficiency due to relatively high inductance of coils takes place.